[RISCV][CostModel] Refine Arithmetic reduction costs (#79103)

This patch is split off from #77342

- Correct for CodeSize cost that 1 instruction is not included. 3 is
from {VMV.S, ReductionOp, VMV.X}
- Add SplitCost
Unordered reduction chain a series of VADD/VFADD/... which scales with
LMUL.
 Ordered reductions chain a series of VFREDOSUMs.
- Use MVT to estimate VL.